Molten metal-air thermal power plant



July 24, 1951 J. F. ALCOCK 2,561,604

MOLTEN METAL-AIR THERMAL POWER PLANT Filed Jan. 13, 1948- m MMv-W Attorney tubesof, the heat exchanger.

difference, however, must be substantial in order Patented July 24, 1951 2,561,604 I C E v "MOLTEN METAL-AIR THERMAL POWER PLANT John Forster Alcock, North Lancing, England, as-

,- i .1 signor, by mesne assignments, to Ricardo & Co.

Engineers (1927) Limited, London, England, a

company of Great Britain Application January 13, 1948, Serial No. 1,937 In .Great Britain January 23, 1947 This invention relates to thermal power plants of .the type in which the workingmedium circulates in a closed system in whichis included at least one compressordriven by a, turbine and one or more heat exchangers in which heat is impartedto or taken from the working medium.

The invention relates more especially to the heat exchangersinsuch plants and has for its object to obviate the use of those of the surface typein which, for example, heat is transferred from gases of combustion or other like outside source through metaLwalls to the working medium. In power plants of the above mentioned kind-in'which heat exchangers of thistype have been employed the metal Walls of the input heater wherein heat is imparted to the working medium from an outside source, have been subjected internally to the fullpressure of the working medium. This has given rise to many problems due to the small difierence between the temperature to which the working medium must be heated and the allowable temperature for the This temperature to obtain an adequate heat transfer owing to the low conductivityof the gas constituting the working medium. There is also the danger that owingwto an unforeseen local increase in the heat transference, for instance owing to some variation in the heat supply from the outside source, the temperature of some of the tubes in the heat exchanger may risewith an increase of local heat flow far more than in a steam generator. Thus overheating and damage to the tubes or their equivalent may happen with relatively slight provocation. r 1

According tothe present .invention the heating of the gaseous Working medium is effected by a liquid heat-conveying medium which after being heated from an outside source is brought into direct contact with the working medium in the circuit so that heat is transferred directly to the Working medium from the liquid heat-conveying medium. The heat-conveying medium employed is for example a molten metal or metal alloy. This heat-conveying medium ispreferably heated under a low pressure in a surface heat exchanger outside the closed circuit of the working medium in. the power. plant into which the liquid heat-conveying medium is then introduced under a, substantially higher pressure. After it has given up heat to the working medium the heat-conveying medium is used to take up heat from the exhaust working medium gases leaving a turbine in the plant before the heat-conveying medium is returned for reheating by the outside source. This heating of the heat-conveying medium from an outside source may be effected invarious ways, as for instance by a fluid flowing in a circuit, this fluiditself being heated roams. (01. -59) by direct or indirect contact with combustion gases from a suitablesource of heat.

The accompanying drawing illustrates dif agrammatically and by way of example a hot gas turbine power plant in which the present invention is embodied.

In the construction shown the working medium may be, for instance, nitrogen and the heatconveying medium, for example, may be molten gallium, and in the following description will be referred to as that metal.

.The gallium is contained in a coil of piping A enclosed in a casing B and the coil is heated, for example, by combustion of fuel in a burner C.

Air enters the casing B through a passage B and the hot gases after passing over the coil issue at B Conveniently the entering air and the issuing hot gases are caused to pass through a heat ex-, changer D which, may be of the rotary type wherein the hot gases pass through a matrix of heat-conducting material to which heat is given up, the heat being subsequently given out to the air entering at B as it passes in the opposite direction through the matrix material in the exchanger D.

The molten gallium from the coil A is delivered by a pump A through a suitable nozzle A in the form of a spray which is directed downwards into a chamber E which eiiect functions as a scrubber and for convenience may be referred to as the gas-heating scrubber. The nitrogen delivered by a compressorF driven by the gas turbine G is delivered through the piping H into the lower part of the scrubber E wherein this gas passes upwards and is met by the downwardly directed spray of molten gallium from the jet nozzle A From the upper part of the scrubber E the" heated gas passes through piping E into a separator J wherein is removed any metal spray that may be carried over from the scrubber E. The

ing the compressor F. i

There is a secondscrubber L, which for convenience may be referred to as the gas-cooling scrubber. The nitrogen after. imparting rotation.

to the turbine G is led through piping L into the lower part of the scrubber L wherein'the gas flows upwardly and, is met by a downwardly directed spray of molten gallium issuing through a nozzle M to which is delivered through piping E, for example by gravity the molten metal;

which hasaccumulated at'the bottom. Ofibh' gasheating scrubber This molten gallium is at a The nitrogen partly cooled by the gallium in the 4 .3; In theiityp'elof regeneratorixiescribed -the liquid heat-conveying medium moves in contraflow to the} gas stream in each scrubber, and each part of the -heat-conveyingamedium swings during the cycle over a temperature range nearly equal to v that of the gas which is being heated or cooled.

gas-cooling scrubber L is led'by wayfiof a pime M into a heat exchanger N throughliwhich water;

passes, the water entering at O ahdis's'uing at The temperature of the nitrogen is further reduced in the heat exchanger N'so asto bring'rthisii; temperature down to the lowest practicable' -value before the gas flows through the piping N [to the compressor F.

The molten gallium .from the jet nozzle M is collected at the bottom of the gaSJ-cooling scrubbergL and takenthence through pipingli to'the' upper 'end'of thefc'oil'A whereimthe'galliu'in is again-heated;

In tha'bove'describedapparatus it willbe ap' parent that the input heater A, B wherein the gallium 'or 'other heat-tc'onveying'mediu'm is heat ed functions with 'th'egas-heating scrubber E as J the.- equivalentiof a surface .heat'exchanger, such as'is commonly employed in known plants of this type, for thepurpose of effectingthe initialheatmeter; the working medium from an outside source) The lowe'r'fpartofthe gas-heating scrubbet. E vii-here thehe'at c'onveying"medium collects functions :in combination with thegas cooling scrubber .L 'as a regen'erator'. If "desirable the functions. of the gas-heatingiscrubber E may be divided betweentwo scrubbers'in one of which Theiadvantag'esofa the'rmal power" plant ac-' cording. to i the present invention may be summed up as follows. The heattransfer coefiicientfrom theheating surface of the input heater A, B to the liquidcheat-conveyingt .medium such as gallium,

issolmuch greater thanithegas-tube coefficient in 'th surface type ,inp'ut heaters-"employed in known thermal power'plants' operating on agaseous medium; thatthe tube "or wall temperature willilbel-practically thatiof the liquid 'heat-convey'ingcmediumv Consequently: accidental local over-heating of thelmetal"walls'will have comparatively .little' iefiect on'.the""temperature of th'esa wallsl'l .This :i-minunity 'from over-heating enablesthe heatingsurfa'ces'of the input heater A to .be e-reduced .by making full use of radiation from. thI-products "of combustion" from the jet nozzleCl "Since in the'coil'A of th'e'input heaterthheat-conveying"mediumis heated at low pressure, ascompared' withthepressu're of upwards of'zofa'tmospheresin known thermal plants using a gaseous mediumfthermetal in the heater "is onlys'liglitly stressed.

Thuse of scrubbers such'as "E and provides in'a small space a" large'surface 'for' heat transfer between the liquidl'heat-conveying medium and thTgaseous workihgmediumr The operation of thej' scrubbers' is facilitated'bythe fact that the possible liquidswhich maybe employed are generally of highdensity, a fact'wh'ich increases the permissible "throu'gh put fora given scrubber towers .Thiscrubbirs "replace" the'normal eX- haiist-etb-compresse'd gas heat exchanger," as well asl'thinput gas 'heater." Apart from the input heater-A the'only tubular or plate surface heat exchangers retainediin 'thlpresent'system are th lwater=cooled heattexchanger N andany in-- tercool'enwhich it 'may'be desirable to employ, and'these may besmall lowtemperature units.

scrubbing and'spray separations 7,0

Thuswhat may bereferred to as the matrix flowratei: is far less: than in the normal regenerator wherethematrix material in effect flows across the'gas stream and for efficient operation canionlyhbelallowed to swing through a small temperature. range. In other words, the quantity of liquid to be pumped is small and there is obviously'no blow-down loss with this type of regenerator. Reheating can be effected easily by feeding partof 'thehot liqiii'dvto a secondsrflbbe'r betweenhigh pressureandilowpressure:tur bin'eswhenthes'e are employed.

In addition to these advantages the--- systemt.- forming-the subject of the *presentinvention retains those which are'inhe'rent to the closedfcycle'; air'turbine power'plant. V I

With" regard to the liquid heat 'conveying mes diu'm; generally speaking the requirements are as follows but'it is to be understood that 'insoma cases it is practicable to use-a liquid heat cbn veying medium which does-"not fulflll' all these requirements. 1 p (1) It shall-not attack themetalofthe'input heater up to temperatures'of thiordefof-WSO C2 (2) It shall not decomposeup'to a temperaturei of the order of 750C.

(3) 'It "shall have no reaction with-ftheworke ing"fluid;'which is preferablyjnitro'genr" V (4) It shall have a low melting-point; prefer? ablyof the'orderof 75 "C. or less-for reasonable T ease" in "starting 'up the plant. This upper "limit to the melting point is 'desirable' fto allow 'for' thawing the system bythe' aidofsteam after 'a' '-shut-down;'-ofthe plants (5) The medium must not1'be inflammable or toxic." (6) It is desirablethat the liquid s'hall have: a high boiling' point to minimise "vapour carry= -over:'- A boiling-point of theordenof 900 'C. -or over 'is'desirable. (7)" A high thermal conductivityiis desirable? (8)--A high density is desirable 'to facilitate Possible materials for employment-in a plant of this nature are: (a) metals, (b) saltsIIAmongs. the metals is Y Gallium "metal; 'its' advantages being:

Melting point of the .orderlof 309:0. Boiling. point of .thel.ordercof "18003.13. Density aboutfi l Tendency to oxidize,.-low. I

Other possibilitiesare metal all'oysyfor' ex-' 'ample,;fusible alloys similaitoWoods metal?- Again, alkali metals, notably sodium "and p0 tassiumgmay be used'since these" have the advantageof a low costand thatthey do not-at tack ferrous'metalsz- Alloys of these two-metals have suit'ablemelting points and for example an alloy "which may-be used may-contain to" of sodium with 50% to40% potassium and havin =a melting point about'20" C. Though"a sodium-potassium alloy does 'not 'actually fulfillf all the above mentioned requirements, on the other-handsuch-an' alloy may be employed. It is undesirable to employ fusible-"metal alloys containing mercury owing --to itsvolatility', or those with a high bismuth content owing to ex-* pansion "on "solidification with consequent of pipe fractures if the plant is allowed to cool off.

If a substance is employed which expands on freezing the ill-efiects of the expansion may be avoided, for example, by either of two methods:

(a) The insertion into the pipes of an inner flattened pipe of thin metal which can absorb the expansion without over-stressing the main pipe or permanently deforming the flattened pipe. In cases where such an inner pipe is employed it may also serve to carry steam for the purpose of thawing the plant in the event of a shut-down.

(b) The provision in the 'plantof means by which all the metal. forming the liquid heatconveying medium may be drained into a sump provided with steam heating coils to enable it to be re-melted on restarting the plant.

With regard to the gaseous working fluid, air is undesirable owing to its liability to cause oxidation of the liquid heat-conveying medium and/or the metal walls of scrubbers and pipe work. Argon would be suitable, but is hardly practicable owing to its cost, as a certain amount of leakage of the working fluid would be inevitable. Nitrogen whichis readily provided is a suitable and convenient gas.

What I claim as my invention and desire to secure by Letters Patent is:

l. A thermal power plant comprising in combination a working fluid circuit containing a gaseous WOIkiIlg medium and including in the circuit a compressor, a turbine which drives the compressor, a delivery passage between the outlet of the compressor and the inlet of the turbine including a first heat exchanger, a return flow passage between the outlet of the turbine and the inlet of the compressor and including a second heat exchanger through which the gaseous working medium passes after leaving the turbine and a third heat exchanger through which the working medium passes after passing through the second heat exchanger, means for passing a cooling fluid through the third heat exchanger to cool the working medium passing therethrough, and a, heat-transfer-liquid circuit in which flows a liquid heat-conveyin medium including the first heat exchanger through which the heat-conveying-liquid flows in the opposite direction to the flow of gaseous working medium therethrough whereby the gaseous working medium leaves the said first heat exchanger at a temperature higher than the heat-conveying liquid leaves the said first heat exchanger, the said second heat exchanger through which the heat-conveying liquid flows in a direction opposite to the working medium flowin therethrough, and a liquid-heating device deriving heat from a source external to the circuits and through which the heat-conveying liquid'passes on its way from the second heat exchanger back to the first heat exchanger, the drop in temperature of the working medium in its passage through the turbine being less than the difference between the temperatures respectively of the working medium and of the heat-conveying liquid leaving the first heat exchanger.

2. A thermal power plant as claimed in claim 1 in which each of the first and second heat exchangers comprises a scrubber operating as a heat exchanger in which heat is transferred between the heat-conveying liquid and the gaseous working medium by direct contact while the liquid and the medium are flowing in opposite directions through the scrubber.

3. A thermal power plant as claimed in claim 2, in which the heat-conveying liquid is heated by the heating device while the said liquid is at a relatively low pressure and means are provided for raising the pressure of the heat-conveying liquid after it has been so heated and causing it to flow at a substantially higher pressure into the first heat-exchanger.

4. A thermal power plant as claimed in claim in which the heat-conveying liquid is a molten metallic mass.

5. A thermal power plant as claimed in claim 1, in which the heat-conveying liquid passes at relatively low pressure through the heating device and means are provided for raising the pressure of the heat-conveying liquid and causing it to flow into the first heat-exchanger at a substantially higher pressure than that at which it is heated in the heating device.

6. A thermal power plant as claimed in claim 5, in which the heat-conveying liquid is a molten metallic mass.

7. A thermal power plant comprising in combination a circuit containing a gaseous working medium and including in the circuit a compressor by which the gaseous medium is caused to flow round the circuit, a turbine which drives the compressor, a delivery passage connecting the compressor outlet to the turbine inlet and including a first scrubber operating as a heat-exchanger through which the working medium passes before it acts on the turbine, a second scrubber operating as a heat-exchanger through which the working medium passes after it has passed through the turbine, a return flow passage between the turbine outlet and the compressor inlet including a surface heat-exchanger through which the gaseous medium passes and in which it is cooled after leaving the second scrubber and before it again enters the compressor, a liquid heat-conveying medium circuit including the first scrubber through which the heat conveying liquid is caused to flow in an opposite direction to and in direct contact with the gaseous working medium, the second scrubber through which the heatconveying liquid flows after leaving the first scrubber and in a direction opposite to and in direct contact with the gaseous working medium, a surface heat-exchanger through which the heat-conveying liquid flows after leaving the second scrubber and in whichheat is imparted to it from an external source of heat while the heat-conveying liquid is at a relatively low pressure, and a pump operative on the heat-conveying liquid when it leaves the said surface heat-exchanger, for delivering it to the first scrubber, the temperature drop of the working medium in passing through the turbine being less than the difference in the temperatures: respectively of the heat-conveying liquid and the working medium leaving the first scrubber.

JOHN FORSTER ALCOCK.

REFERENCES CITED UNITED STATES PATENTS Name Date Turner Mar. 28, 1939 FOREIGN PATENTS Country Date Great Britain Jan. 2, 1942 Number 2,151,949

Number 

